114 THEORY OF COLLOIDAL BEHAVIOR 



Moreover, the attempt to explain the depressing effect of the 

 addition of salts on the basis of the micella theory fails com- 

 pletely in the case of the other properties of protein solutions 

 which are equally depressed by them as the osmotic pressure, 

 namely, the viscosity and the P.D. 



We shall see in Chap. XIII that if the state of aggregation 

 increases in a gelatin solution i.e., if isolated protein molecules 

 or ions unite to form a larger aggregate the viscosity of the 

 solution is thereby increased, for the reason that these aggregates 

 occlude comparatively large quantities of water whereby the 

 relative volume occupied by the gelatin in the solution is increased. 

 This increase in the volume of the micellae at the expense of 

 water leads, as will be seen, to an increase in viscosity. Hence, 

 if we assume that the addition of a salt increases the degree of 

 aggregation in protein solution, it would follow that this should 

 result in an increase of viscosity; while the addition of salt 

 depresses the viscosity. The attempt to explain the depressing 

 influence of salts on the osmotic pressure and viscosity of protein 

 solutions on the basis of the aggregation theory leads therefore 

 to conclusions which are in contradiction with the actual facts. 



An attempt to account for the colloidal behavior of protein 

 solutions was made by Pauli in his theory of hydration of protein 

 ions. 



Kohlrausch had tried to account for the differences in the 

 mobility of different ions by the assumption that each ion is 

 surrounded by a shell of water and that the velocity of migration 

 of ions is greatest where this shell of water is a minimum. Pauli 

 assumes that the protein ion is surrounded by an enormous 

 shell of water while no such jacket of water surrounds the 

 non-ionized protein. 1 The shell of water might prevent the coa- 

 lescence of the protein ions and, hence, might cause a higher 

 degree of dispersion. On the basis of the hydration theory we 

 can find a qualitative explanation of the peculiar pH curves in the 

 following way : At the isoelectric point protein is in a non-ionized 

 condition and no hydration occurs. Hence, the degree of disper- 

 sion of particles and the osmotic pressure are a minimum at this 

 point and the viscosity and swelling should also be a minimum, 



1 PAULI, W., Fortschr. naturwiss. Forschung, vol. 4, p. 223, 1912, "Kol- 

 loidchemie der Eiweisskorper," Dresden and Leipsic, 1920, 



